Acoustic well logging techniques and tools are extensively described in the art. Acoustic well logging is used to provide surveys of formations traversed by earth boreholes. In particular, measurements are made of the velocities of acoustic waves to reveal valuable information concerning the type of rocks and the porosity of the rocks in the formation surrounding the borehole. A commonly measured acoustic parameter is the velocity of compressional waves. However, other acoustic wave parameters such as the velocity of shear waves and other modes of acoustic energy are also useful in evaluating the formation.
An acoustic well logging tool for measuring the velocity of acoustic waves typically employs a sonic pulse transmitter and a plurality of sonic receivers selectively spaced from the transmitter. The sonic receivers include transducers to convert the incident acoustic wave to an electrical waveform and suitable amplifiers to transmit the waveforms to surface located processing equipment.
The spacings between the sonic receivers and the transmitter influence the character of the parameter measurement. For example, a longer spacing enables a greater effect of the formation characteristics on the sonic wave with the possibility that wave analysis can thus reveal more useful data about the formation. On the other hand, a longer spacing results in the arrival of greatly attenuated sonic waves making their analysis more difficult. When many spaced sonic receivers are employed to measure the velocity of the acoustic wave, the need to maintain adequate spacings between each of these receivers results in strongly attenuated waves.
Current sonic well logging tools commonly use one transmitter with a pair of sonic receivers. A double pair of transmitter-receivers has been used in the art, for example to provide compensation for physical misalignments of the tool in the borehole as described in the U.S. Pat. No. 3,304,537 to Schwartz. A three receiver, single transmitter sonic well logging tool has been described in U.S. Pat. No. 3,390,377 to Elliott et al. Generally, when the number of receivers is increased to improve the velocity measurement, special techniques are needed to process the increase in the flow of signals and enhance the accuracy of sonic wave parameter measurements.
The use of computers in determining any of several correlation functions between two signals is a well established field. Much literature has been written on the subject and general reference may be made, for example, to a book entitled "Statistical Communication and Detection" written by E. A. Robinson and published by the Hafner Publishing Company of New York in 1967. Of particular interest in this book is the section dealing with numerical filtering methods for digital computers. Various correlation techniques are described.
In a well logging technique in accordance with the invention the location of a wave in a waveform can be accurately detected. In accordance with one such technique for the first motion in the waveforms is detected. A gauge value representative of the magnitude of the waveform is determined and used to derive a threshold level. A comparison technique is applied to the waveform to detect the time when the waveform first exceeds the threshold. The threshold automatically varies with the amplitude of the waveform, to enable an accurate determination of the first motion with various waveform sizes.
The technique for detecting the first motion is advantageously applied to sampled stored waveforms. The waveform can be quickly scanned to measure its magnitude. A threshold which may be, for example, a predetermined fraction of the magnitude, is then established to detect the first motion. With such technique for detecting a first motion, an automatic compensation for waveform amplitude variation is obtained.
The first motion detection technique is advantageously applied to determine an acoustic wave parameter such as the transit time of a compressional wave by comparing the first motion detections of waveforms derived from different receivers.
The first motion detection technique may be further advantageously employed to control the start of a multiple fold correlation process to determine the acoustic wave parameter of a sonic wave present in a set of waveforms. The first motion detection determines the location of a first segment for a first waveform. This segment is then correlated with other waveform segments whose positions are varied as a function of a range of values of the acoustic wave parameter. In effect, the time to execute the correlation technique may be advantageously reduced by precisely determining a starting place with the first motion detection.
In another technique for determining the location of an acoustic wave in a waveform a pattern comparison is used to determine where the best similarity between the waveforms occurs. As described with reference to one embodiment the pattern comparison technique includes the detection of the first motion of the compressional wave in each of a plurality of sampled waveforms. Thereupon a comparison window is formed of a predetermined number of waveform samples and a pattern comparison is made between the window samples in the respective waveforms in the vicinity of the first motions.
The comparison windows are relatively aligned for different samples over a preselected range to precisely detect the sample index value yielding the best pattern similarity. The difference between the first motion detections is then corrected for the amount of alignment needed to obtain the best similarity of the patterns to determine the velocity of the compressional wave.
The pattern comparison well logging technique for determining an acoustic wave parameter may be rapidly completed for each set of waveforms to facilitate real time processing of the waveforms with relatively slow but economical processors.
It is, therefore, an object of the invention to provide a method and apparatus for accurately determining the location of an acoustic wave in waveforms representative of sonic waves detected during sonic well logging.
It is a further object of the invention to provide method and apparatus for accurately determining the first motion on waveforms derived with a sonic well logging tool.
The novel features of the present invention are set forth with particularity in the appended claims. The operation, together with further objects and advantages of the invention, may be best understood by way of illustration and examples of certain embodiments when taken in conjunction with the accompanying drawings.